A refrigeration system may comprise, in a known manner:                a circuit for circulating a refrigerant successively including a condenser, an expander, an evaporator, and a compression device connected in series, the compression device comprising at least one first compressor and one second compressor mounted in parallel, each compressor comprising an enclosure having a low-pressure part in particular containing an oil pan arranged in the bottom of the enclosure, a high-pressure part in particular containing a compression stage, a refrigerant inlet emerging in the low-pressure part, and a refrigerant outlet emerging in the high-pressure part,        a refrigerant distribution device comprising a distribution pipe connected to the evaporator, a first bypass pipe putting the distribution pipe in communication with the refrigerant inlet of the first compressor, and a second bypass pipe putting the distribution pipe in communication with the refrigerant inlet of the second compressor,        a refrigerant discharge device comprising a discharge pipe connected to the condenser, a first bypass pipe putting the discharge pipe in communication with the refrigerant outlet of the first compressor, and a second bypass pipe putting the discharge pipe in communication with the refrigerant outlet of the second compressor.        
In order to ensure proper operation and good reliability of such a refrigeration system, it is necessary to balance the oil levels in the pans of the two compressors. This oil level balancing is advantageously obtained by arranging an oil separating device between the condenser and the compression device, by putting an oil outlet of the oil separating device in relation with the oil pans of the two compressors using an oil return pipe equipped with two bypass portions each connected to the oil pan of one of the compressors, and by providing a solenoid valve on each bypass portion arranged to open when the oil level in the corresponding compressor drops below a predetermined minimum value.
In this way, when the oil level in one of the compressors reaches a minimum value, the refrigeration system is arranged to favor a return of oil toward the compressor, so as to ensure a satisfactory oil level in each compressor.
Such a refrigeration system nevertheless has the drawback in particular of requiring the presence of solenoid valves, means for controlling the latter parts, and oil level sensors. This results in a complex, expensive refrigeration system, the reliability of which may be questionable, for example in the event of a failure of the solenoid valves, the means for controlling the latter parts, or the oil level sensors.
Document WO 2009/149726 discloses a refrigeration compressor comprising:                a sealed enclosure containing a compression stage and provided with a refrigerant inlet and a refrigerant outlet, the compressor being configured such that during usage conditions, a flow of refrigerant circulates through the refrigerant inlet, the compression stage, and the refrigerant outlet,        an oil pan housed in the lower part of the sealed enclosure, and        oil recirculation means arranged to orient the oil contained in the oil pan into the flow of refrigerant when the oil in the oil pan reaches or exceeds a predetermined oil level.        
According to one embodiment described in document WO 2009/149726, the recirculation means include a bypass line comprising an inlet port emerging radially in the enclosure of the compressor and situated at a height substantially corresponding to the predetermined oil level, an outlet port emerging in the refrigerant inlet, and an intermediate part connecting the inlet and outlet ports of the recirculation line.
The compressor described in document WO 2009/149726 makes it possible, under certain operating conditions, to circulate the excess oil in the refrigerant flow.
Thus, when a refrigeration system is equipped with a plurality of compressors as described in document WO 2009/149726, each compressor is designed to prevent the oil level in the respective oil pan from exceeding a predetermined value, and therefore to ensure a minimum satisfactory oil level in the other compressors.
However, the structure and arrangement of the bypass line of such a compressor do not make it possible to begin suctioning the excess oil in the recirculation line, when the pressure difference between the inlet and outlet ports of the bypass line is small or when the difference in speed of the refrigerant on either side of the inlet and outlet ports of the bypass line is small.
Thus, under the operating conditions mentioned above, the oil level in one of the compressors may significantly exceed the predetermined oil level, and the oil level in one of the other compressors may thereby drop below a minimum satisfactory level, which may lead to poor lubrication of the moving parts of the compressor.
The present invention aims to resolve these drawbacks.